Generally, Light Emitting Diodes (LEDs) are well-known semiconductor light-emitting devices which convert electric current into light energy. LEDs have been used as light sources for displaying images in electronic equipment such as information communication equipment, etc.
The wavelength of light emitted by such LEDs is dependent upon the types of the semiconductor materials used in the manufacture of the LEDs. This is because the wavelength of emitted light is dependent upon the band gap of the semiconductor materials, where the band gap is an energy difference between valence band electrons and conduction band electrons.
Gallium nitride (GaN) has a high thermal stability and a broad band gap (ranging from 0.8 to 6.2 eV), and therefore has received a great deal of attention in developing high-power output electronic component devices including LEDs.
One of the reasons why gallium nitride has attracted a great deal of interest is because it is possible to fabricate semiconductor layers emitting green, blue and white light, by using GaN in combination with other elements such as indium (In), aluminum (Al) and the like.
Due to the capability to control an emission wavelength via the use of GaN, the emission wavelength may be adjusted to a desired range suited for intrinsic properties of the used materials in compliance with the characteristics of specific equipment. For example, the use of GaN makes it possible to manufacture blue LEDs which are beneficial for optical writing and white LEDs which may be substituted for incandescent lamps.
Due to various advantages of such GaN-based materials, the GaN-based LED market is rapidly growing As a result, technologies of GaN-based optoelectronic devices have been rapidly advancing since their commercial introduction.
The technique of fabricating a white light emitting device using the above GaN-based LED is divided into two methods. In the first method, white light is obtained by depositing a phosphor onto a blue or UV light emitting diode chip. In the second method, white light is obtained by combining output from three different color output LED chips.
Further, the luminance of the output of the above GaN-based LED depends on the structure of an active layer, efficiency in extracting light to the outside (i.e., light extraction efficiency), the size of an LED chip, the kind and angle of a mold or resin layer when a lamp package is assembled, phosphor used, etc.